Phase-locked loops (PLL) are widely used in telecommunications, computers and other electronic applications. With the development of the semiconductor industry, operation frequencies of microprocessors have been even higher than ever. A PLL is designed in a microprocessor as the system synchronizer as well as the frequency integrator, to eliminate any inconsistence in terms of timing between the external reference clock and internal clock, and to support the need of an internal high frequency clock. On the other hand, the PLL is much needed for system synchronizing, clock or data recovering, and frequency integration, in a communication system. PLL is important in a vast number of applications for system design.
Although being widely utilized, traditional phase-locked loops are digital PLLs containing the phase frequency detector(s), analog loop filter(s), and voltage controlled oscillator(s). Under the stream of the system on chip (SOC) design, issues regarding the integration of digital and analog circuits such as signal disturbing to the analog circuit will sure be encountered if the traditional PLL circuit design is adopted. Therefore, the all-digital phase-locked loop (ADPLL) has been prevailed in the market for application.
In general, basic circuit design of a typical ADPLL can be illustrated as FIG. 1 shows. As shown in FIG. 1, the ADPLL 1 is consisted of a phase frequency detector (PFD) 11, a digital loop filter 12, a digital controlled oscillator (DCO) 13, and a frequency divider 14. The PFD 11 compares a signal Fi from the DCO 13 with an outer reference signal Fr, and then outputs a series of either increment or decrement signals based upon the frequency as well as the phase difference between Fi and Fr. The loop filter 12 receives the signal series described above and converts the input into analog signals acceptable to the DCO 13, to adjust both the frequency and phase of the output from DCO 13. The DCO 13 is an oscillation circuit generating a relative oscillation frequency depending on the voltage of the analog signals, while the value of the frequency is within a certain range. On the feedback path of the circuit, the frequency divider 14 reduces its input signals by N (N is a natural number) times.
The operation process of the above ADPLL is summarized as follows. The PFD 11 compares a reference clock signal Fr with a feedback clock signal Fi from a frequency divider 14 (the frequency of the feedback signal has been reduced by N times) on either positive or negative edges. Decrement signals will be generated if the edges of Fi lead the edges of Fr. On the other hand, increment signal will be generated if the edges of Fi fall behind those of Fr. And then the digital loop filter 12 generates a control signal to the DCO 13, based on the input from the PFD 11. According to the control signal, a feedback signal is generated from the DCO 13 and passes through the frequency divider 14 which reduces the frequency by N times. The frequency-divided feedback signal Fi will then be compared with Fr as a routine, to continuously modify the voltage level of the control signal from the DCO 13 until the frequency and phase difference between Fr and Fi is minimized. When the locking is effective, either the decrement or the increment signals from PFD 11 shall be zero.
Traditional analog PLLs require accurate analog and passive devices such as resistors and capacitors, and they are sensitive to the process, voltage, and temperature (PVT) variations. Compared with analog PLLs, the all-digital PLL (ADPLL) has advantages such as robustness, easy-to-process migration, and without a passive loop filter. But the oscillator in the analog PLL has a higher operation frequency and better jitter performance than those of the digital controlled oscillator (DCO) in the ADPLL. Moreover, ADPLL has the problems of finite frequency resolution and quantization noise. To have a wider range of operation frequency, the algorithm in an ADPLL has to shorten the locked time. To have a large multiplication factor, the frequency resolution must be high enough to reduce the frequency error and jitter issue. Unfortunately, due to a limited bandwidth (490 MHz to 1.39 GHz), outputs of the current digital controlled oscillators still cannot be broadly applied to the current wireless communication facilities (15 KHz to 1.39 GHz). Therefore, the need for a new ADPLL with an increased DCO operation frequency range and enhanced frequency resolution is urgent.